U.S. Pat. No. 6,706,416 and a number of earlier patents referred to therein, including U.S. Pat. No. 5,051,315, all to Cacace and referred to herein as the “earlier Cacace” patents and processes, deal with a method of producing SS-clad steel products. In these earlier Cacace patents, a billet is prepared by progressively compressing carbon steel swarf into briquettes in a round SS tube. On perusal of these patents it seems clear that the achievement of a satisfactory metallurgical bond at the interface between the SS cladding and the core of briquetted swarf has been problematical. The root of the problem is the occurrence of oxidation of chrome in the stainless steel at the interface. U.S. Pat. No. 6,706,416 discloses a process of dealing with this oxidation. This is possibly the reason that, as far as the applicant is aware, the process is the only one for producing SS clad reinforcing bar that has been in recent commercial use.
There are significant disadvantages to the use of a billet that is produced by compressing swarf into the SS tube. The production of such a billet would require costly specialised machinery, an example of which is described in U.S. Pat. No. 5,088,399 (one of the earlier Cacace patents). Most modern mills cannot roll round billets. They are designed to roll square billets that can be up to 15 m long and are typically 130 mm to 150 mm in cross sectional size. The billets described in the earlier Cacace patents are round and are about 2 m long×100 mm in diameter. This is likely to be close to the maximum size that machinery for producing billets by compressing swarf into a tube can deal with. Only a limited number of existing rolling mills are able to roll billets of such short length and there are even fewer that can also roll from a round billet. A major advantage of the present process is that the billet size can be chosen to suit an existing rolling mill.
Although in principle the size and length of billets that comprise swarf compressed into the SS tube could be increased, and the shape changed, the technical problems involved in achieving suitable machinery for this purpose might well be insuperable.
It is one object of the invention to provide a method that enables billets to be produced whose length, size and shape enable them to be rolled in existing rolling mills.
International patent applications no. PCT/GB2010/001932, PCT/GB2010/001933 and PCT/GB2010/001934 to Cacace (which will be referred to herein respectively as “Cacace 1932”, “Cacace 1933” and Cacace 1934″) deal with methods of producing a composite billet consisting of an alloy cladding on a solid steel core. The technology disclosed in these applications is directed to dealing with problems and disadvantages associated with methods of producing such products disclosed in various earlier patents.
GB 2085331 (Sumitomo), EP 225983 (Mitsubishi) and EP 59070 (Spencer Clark) each disclose methods of producing clad steel products from billets comprising a solid steel body in a SS tube. However, none of these disclosures make any reference to the aforementioned problem of oxidation of the chrome in the SS. The applicant believes that such oxidation would be seriously detrimental to the bond between the core and the tube in the finished product and would result in a commercially unacceptable clad steel product.
In Cacace 1932/3/4, techniques are disclosed for preventing this oxidation. The tube projects clear of each end of the solid core bar and the techniques envision the placement of a mass of finely divided scavenging metal such as aluminium, titanium or magnesium in the tube adjacent the ends of the core bar. The ends of the tube are sealed to prevent oxidising gases from entering the billet when it is heated and rolled. The technique proposes heating the ends of the billet before the core is heated so that the scavenging metal becomes active to scavenge residual oxygen in the billet before oxidation of the chrome in the tube can occur.
Notwithstanding the provision of the scavenging metal, it is stated that it is necessary to ensure that the tube fits closely around the core. To this end, the tubes disclosed in Cacace 1932/3/4 can take various forms. In Cacace 1933, a prefabricated tube is used into which a core bar is inserted. The tube is then stretched beyond its elastic limit which has the benefit of permanently increasing the length of the tube and at the same time reducing the gap between the tube and the core bar. However, the apparatus required for stretching on a commercial scale is likely to be relatively expensive. Furthermore, the tube needs to have portions at its ends that must be flared to enable them to be gripped by the stretching apparatus. These portions increase the length of the portions of the tube that are cut off after use and thus wasted. This adds to the expense of the billet, especially considering the high current cost of the alloys contemplated herein.
Another disadvantage of stretching the tube over the core bar is that a tube so stretched has only a limited ability to take up irregularities in the shape, particularly in the cross sectional profile, of the core bar. This disadvantage is significant when such objects as the so called ‘near net shape’ continuously cast billets or blooms produced, typically, by Siemens VAI Bloom-Beam Blank Casting Technology, or even used axles or the head, flange, or web portions of used rails, are used for the core bar. The same disadvantages could apply to a core that is fabricated from one or more elongate components packed together so as to result in an irregular or asymmetrical cross sectional shape.
In Cacace 1932/4, each tube can be fabricated from one or more elongate components that are welded together after being placed around the core bar. Where the core bar has a square cross sectional profile, the components may be flat or may be square tubes, angle sections or channel sections shaped so as to conform to part or whole of the profile.
It is one object of the present invention to provide an alternative technique for reducing the gap between the tube and the core bar.
Cacace 1932/3/4 propose that, in some cases, the entire tube, including the projecting ends, is of SS or one of the other alloys contemplated herein. However, these alloys are costly and it is desirable to minimise the quantity thereof that is used to make up a tube. To this end, in some cases, the alloy portion of the tube terminates short of the ends of the core bar and the ends of the tube are made up of steel sleeves which are welded to the alloy portion.
The steel sleeves proposed in Cacace 1932/3/4 reduce the amount of alloy that is discarded in this manner. However, the welds that join the steel sleeves to the alloy portion, and indeed welds anywhere in the tube, particularly near its ends, are a potential source of weakness in the tube. This is also true of the welds that fix in place closing plates are inserted in the overlying ends of the tube and welded in place to close the ends of the tube. When closed by such plates, the tube, whether or not it comprises the steel end pieces, forms a closed housing in which the core bar is sealed. The most common cause of oxidation of the metals at the interface has been found to be failure of such welds which are particularly vulnerable in billets which are not tapered at their ends. As is well known, so called “fish tails” are commonly formed in the ends of billets having non-tapered ends during rolling.
U.S. Pat. Nos. 5,051,315, 6,706,416 and Cacace 1934 propose that the tube ends can be “closed” using an apparatus that “crimps” each end into a star shape. Although the procedure reduces the size of the tube ends, thus facilitating entry of the billet into the rolls and reducing the possibility of weld failure in the proximity thereof, the tube ends are left with a star shape. Furthermore, it is clear from a perusal of each document that the tube ends are left substantially open in the heating furnace preparatory to rolling, and not sealed from the atmosphere by this procedure. In U.S. Pat. No. 6,706,416 reliance is placed instead on the presence of two additives in the swarf of the core, usually aluminium and ammonium chloride, to prevent oxidation of chrome in the alloy as aforesaid. The techniques disclosed in Cacace 1934 render the use of ammonium chloride unnecessary.
As described in detail below, the applicant has now found that significant and unexpected benefits arise from causing the tube ends to be tapered to a uniformly round or square shape and it is one object of the invention to achieve this end.
It should be clear that many descriptive terms found herein are used in the sense in which they are used the steel industry. Persons skilled in the art will thus be aware that, for example, a metal tube or bar that is described as ‘square’ will inevitably have corners that are rounded to some extent. For many purposes, metal bars and tubes are purposely formed with round corners and, to be commercially acceptable, many of the characteristics of such products, including the radius applied to the corners of bars and tubes, will be governed by authoritative specifications. To avoid excessive repetition, the following terms have the meanings indicated unless it is clear from the context that this is not intended:                “square bar” and “rectangular bar” include such bars having corners that are rounded in the course of production;        “square tube” and “rectangular tube” include square tubes and rectangular tubes having corners that are rounded in the course of production.        
Further, although some of the descriptions are based on the use of a round cornered square bar inserted into a round cornered alloy tube, they could equally apply to the use of bars and tubes of any other suitable shapes including both round and out of round shapes, due account being taken of the differences in shape.